It is historic, but only because it has not happened before. The agreement to permit televised election debates between political leaders does not automatically enhance British democracy. It simply raises the possibility that democracy may benefit if a series of crucial details are pinned down to the advantage of the electorate.

Supporters of televised debates like to cite evidence from the US, where research from the 1992 and 1996 presidential debates suggests voters found them valuable. Broadcasters and audiences alike remember key moments in the gladiatorial contests between candidates for the leadership of the free world. Older electors recall Nixon looking tired while JFK exuded youth and sophistication. Others remember Bill Clinton stepping out from behind the podium in 1992 to make George Bush Sr look wooden and remote.

But these comparisons are barely relevant. Those were head-to-head contests between candidates seeking direct personal mandates. British party leaders are not in that category. They are candidates in individual constituencies for whom only their own constituents can vote. To become prime minister they depend upon party success and party consent. It is for this reason that some purists condemn debates between leaders as unconstitutional in our party-based democracy.

That objection is too abstract for most tastes. But it does suggest a better comparison than the American one. Scotland provides it. Televised debates were held during the first elections to the Scottish parliament in 1999 and they have continued since. Key lessons have emerged. First, without a remarkable presenter and copious luck such debates make stultifying viewing. Second the party leaders approach them as exercises in damage limitation, not as chances to inspire and enthuse.

Will the UK versions be better? The three-way format is a mistake. In Scotland the semi-proportional voting system and a recent history of coalition government makes participation by the Liberal Democrats and Scottish Conservatives relevant. Clegg's participation on the BBC, Sky and ITV is harder to justify. The electorate would be better served by at least one blunt confrontation between Brown and Cameron, the only genuine contenders for the premiership under existing electoral rules.

At their best, leaders' debates provide a cockpit in which the electorate auditions candidates for national leadership. But for that to happen the format must be open and the subject matter determined by the electorate. Australia, a parliamentary democracy with experience of live televised debates, offers blunt guidance: audiences rise when real voters set the agenda.

For that to happen another golden rule applies: the presenters must be as free of personal ego as that occupation permits. Sky's Adam Boulton, a political specialist employed by a broadcaster dedicated to audience engagement, can thrive. The BBC may regret its choice of David Dimbleby. I would deploy 5 Live's superb Peter Allen. His CV and on-air persona make the case for him, and he is a stranger to pomposity.

Alastair Stewart's acceptance of an OBE marks him out, for me, as an establishment insider who will only reluctantly challenge, confront and hold aspirants to power ruthlessly to account. But he is capable of rising to the challenge.

Will I watch? Certainly. In a democracy it is only reasonable to require candidates for the leadership of government to demonstrate their mettle before those they aspire to govern. But beware the sanctimony of MPs who see this as a chance to repair the damage done by their expenses claims.

To promote renewed democratic vigour Brown, Cameron and Clegg must do more than just appear. They should accept their duty to answer any question the electorate throws at them. And the broadcasters must act as pure facilitators, not ratings-chasing competitors. High hopes then.